Ultrasonic transducers operating into liquids, solids and gas have found many applications in signal processing devices such as delay lines, resonators, convolvers and correlators, and in systems for medical imaging, non-destructive evaluation, and underwater sensing.
The generation of ultrasound in air is of interest for a variety of applications including robotic distance sensing and imaging, gas flow measurements, in-situ process monitoring and acoustic microscopy. To achieve maximum resolution, it is necessary to operate at the highest possible frequency. This is limited by the attenuation of sound waves in air which is about 1.2 dB/cm at 1 MHz, and increases as the square of the frequency. Of particular interest is operation in the 200 kHz to 10 MHz frequency range.
Conventional techniques to generate sound at these frequencies use a piezoelectric material (typically lead-zirconium-titanate). These piezoelectric materials have an acoustic impedance of approximately 35 MRayls (kg/m.sup.2 s), while air has an impedance of 0.0004 MRayls. Thus, there is an impedance mismatch of 5 orders of magnitude. This results in extremely poor efficiency and bandwidth. In practice, one or more quarter-wave layers of material are used to match the impedance of the piezoelectric material to air. Because of the low bandwidth, prior art transducers are very dependent on the attenuation of the matching layer. Materials for matching layers which have the right acoustic impedance, low attenuation and are easy to work with are not available. Thus, efficient high-frequency ultrasonic air transducers are not available.
Electrostatic transducers have been used for transmitting, emitting and receiving acoustic energy. For example, electrostatic transducers have long been used for audio speakers and capacitance receivers. Large-area electrostatic transducer arrays have been used for acoustic imaging. Electrostatic transducers work on the principle of electrostatic attraction between the plates of a capacitor. The attractive force is proportional to the square of the electric field. As the plates are moved by the application of an electrical signal, they generate sound at that frequency. When sound is received by the transducer, it can be used to generate a corresponding electrical signal.
Electrostatic transducers have also been used for ultrasonic application. Electrostatic ultrasonic sources have typically relied on roughening the surface of a metal plate and bonding a thin metalized dielectric to the plate. The microscopic grooves in the plate act as resonators and determine the frequency response of the transducer. These devices are not easily characterized and their fabrication is more art than science.
There is a need to provide electrostatic transducers which can efficiently generate and receive ultrasound in air over a broad band of frequencies.